Injection Molding of Crosslinking Polymers

ABSTRACT

Non-time dependent measured variables are used to effectively determine an optimal hold profile for an expanding crosslinking polymer part in a mold cavity. A system and/or approach may first inject molten expanding crosslinking polymer into a mold cavity, then measure at least one non-time dependent variable during an injection molding cycle. Next, the system and/or method commences a hold profile for the part, and upon completing the hold profile, the part is ejected from the mold cavity, whereupon a cure profile is commenced.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/520,004, entitled “Injection Molding of Crosslinking Polymers”, filedJun. 15, 2017, the entirety of which is incorporated by referenceherein.

FIELD OF THE DISCLOSURE

The present disclosure generally relates to injection molding and, moreparticularly, to injection molding of expanding crosslinking polymers.

BACKGROUND

Injection molding is a technology commonly used for high-volumemanufacturing of parts constructed from thermoplastic materials. Duringrepetitive injection molding processes, a thermoplastic resin, typicallyin the form of small pellets or beads, is introduced into an injectionmolding machine which melts the pellets under heat and pressure. Themolten material is then forcefully injected into a mold cavity having aparticular desired cavity shape. The injected plastic is held underpressure in the mold cavity and subsequently is cooled and removed as asolidified part having a shape closely resembling the cavity shape ofthe mold. A single mold may have any number of individual cavities whichcan be connected to a flow channel by a gate that directs the flow ofthe molten resin into the cavity.

Expanding crosslinking polymers (e.g., ethylene-vinyl acetate or “EVA”)are one class of polymers that are commonly injection molded. A typicalinjection molding process of expanding crosslinking polymers generallyincludes four basic operations. First, the plastic is heated in theinjection molding machine to allow the plastic to flow under pressure.When injection molding expanding crosslinking polymers, at this step,the polymer is heated to a temperature that is below an activationtemperature of the polymer, or the temperature at which expansion andcrosslinking within the polymer begins to occur.

Next, the melted plastic is injected into a mold cavity or cavitiesdefined between two mold halves that have been closed. The mold orcavity temperature is set to a value that is high enough to activate achemical reaction or reactions that cause the polymer to begin expansionand crosslinking. At a third step, the plastic is held under pressure toallow adequate crosslinking and expansion (or blowing) to occur in thecavity or cavities. Last, the mold halves are opened, and the moldedarticle is removed or ejected from the mold, thereby allowing theplastic to expand to a final shape and configuration that is larger thanthe internal volume of the mold cavity.

In conventional systems, a fixed, predetermined volume of plastic isinjected into the mold cavity. This volume only partially fills thecavity. The mold cavity is then heated to cause a chemical reaction,upon which the plastic is then left to expand to fill the mold cavityand crosslink for a specified hold time, which is typically determinedvia a “gate freeze study” where the part weight is measured over aperiod of time. In this gate freeze study, part weights are periodicallymeasured during the injection molding process until the weight begins tolevel off. The point at which the part weight levels off is identifiedas generally being the optimum time to eject the part. This gate freezestudy is typically performed during a process validation stage, and isoftentimes used for the entirety of subsequent injection molding cycles.

After the part is ejected, it is quickly removed from the mold to astabilization tunnel where curing occurs. By quickly removing the partfrom the mold, the part can fully expand, and will not be deformed dueto the material being constrained from expanding at areas where the partis still captured in the mold. During the curing phase, the part isallowed to slowly cool to a temperature near room temperature. Excessinternal gases will slowly escape from the part.

The time when the plastic is ejected (which is dependent on thecalculated hold time) is determined or calculated to provide theinjected plastic sufficient time to expand and crosslink (thus beingsufficiently hardened) to the desired final shape so the plastic doesnot deform or become otherwise damaged. However, due to material andmachine variances, using a fixed hold time as the determining variablecan result in varying internal peak cavity pressures, which can impactcrosslinking and expansion while in the mold cavity. Specifically, thechemical reaction that causes the part to expand is less consistent, asevidenced by both delayed and inconsistent pressure-builds in existingsystems. In turn, when the part is ejected from the mold and enters acuring stage where the molded parts continue to expand and crosslinkuntil reaching a final form, expansion and crosslinking may occur atvarying rates, thus resulting in inconsistently sized parts. Further,the parts may have unsightly blemishes and other undesirable flaws.

For example, a melted plastic may have slightly different materialcharacteristics in subsequent injection cycles, thus if subsequentinjection cycles were to depend on prior hold times, the occurrence ofpart imperfections, faults, and other irregularities may arise. If apart is held in the cavity longer than needed, the overall injectionmolding cycle is unnecessarily long, thus the injection molding machineconsumes excess energy which in turn increases operating costs andadversely impacts production capacity. Further, the molded parts may notexperience consistent heat transfer in the mold, which can result in anon-uniform skin layer. The cell structure of the molded part may alsobe non-uniform, meaning free radical molecules may not be aligned. Whenthese molecules are uniformly distributed, the resulting part has moreconsistent and stable dimensions and physical properties.

Further, conventional systems typically do not provide uniform heatdistribution throughout the plastic during the molding process due tovarying mold thicknesses. By unevenly heating the plastic, differentregions of the plastic within the mold cavity can expand at differentrates, which can result in inconsistent parts having wide tolerances.

Further, the molded parts may be incorrectly dimensioned (meaning, partsmay be either too large or too small) and may potentially be too soft ortoo resilient due to insufficient crosslinking. As a result, the moldedpart may fail any number of objective tests such as an abrasion test, acompression set test, and/or a dynamic elasticity test where energy lossis measured over a number of closely timed impacts with a controlledload.

SUMMARY

Embodiments within the scope of the present invention are directed tothe use of non-time dependent measured variables to effectivelydetermine an optimal hold profile of one or more expanding crosslinkingpolymer parts being formed in a mold cavity. A system and/or approachmay first inject molten expanding crosslinking polymer into a moldcavity, then measure at least one non-time dependent variable during aninjection molding cycle. Next, the system and/or method commences a holdprofile for the part, and upon completing the hold profile, the part isejected from the mold cavity, whereby the system and/or method commencesa cure profile for the part.

In these examples, the mold cavity is nearly completely filled at aninjection stage. A suitable hold profile commences when at least onemeasured non-time dependent variable reaches a first threshold value,and continues until the measured at least one non-time dependentvariable(s) reaches a second threshold value. During this period,additional molten expanding crosslinking polymer is restricted frombeing injected into the mold cavity.

In some examples, the first threshold value is indicative of the moldcavity being substantially full of molten expanding crosslinkingpolymer. The second threshold value may be indicative of the part beingstructurally sound, and being ready to be ejected.

In some examples, the measured variable is a cavity pressure value. Inthese examples, the first threshold value may be a nominal increase incavity pressure. The second threshold value may be indicative of asubstantially constant cavity pressure value over a specified period oftime. Other examples of threshold values with respect to cavity pressuremeasurements are possible.

In other examples, the measured variable is a temperature value. Inthese examples, the first threshold value may be a nominal increaseabove an initial cavity temperature. The second threshold value mayrepresent a substantially constant cavity temperature value over aspecified period of time. Other examples of threshold values withrespect to cavity temperature measurements are possible.

In some examples, commencement of the cure profile includes first,measuring a different non-time dependent variable. Upon the measureddifferent non-time dependent variable reaching a third threshold value,the cure profile is ended. In these examples, the third threshold valuemay be indicative of the part being structurally sound. The measureddifferent non-time dependent variable comprises a pressure value whichmay be measured directly via a pressure transducer, via sub-surfacemeasurements, and/or via an indirect measurement. Other examples arepossible.

In other examples, commencing the cure profile includes allowing thepart to cool for a predetermined amount of time.

In some approaches, an expanding crosslinking polymer injection moldingsystem includes an injection molding machine comprising an injectionunit and a mold forming at least one mold cavity, a controller adaptedto control operation of the injection molding machine, and one or moresensors coupled to the injection molding machine and the controller. Theinjection unit is adapted to receive and inject a molten expandingcrosslinking plastic material into the at least one mold cavity to forma molded part. At least one of the one or more sensors is adapted tomeasure at least one non-time dependent variable during the injectionmold cycle. The controller is adapted to commence a hold profile for theexpanding crosslinking polymer part, and is further adapted to cause themolded part to be ejected from the mold cavity upon completing the holdprofile, whereupon a cure profile then commences.

By optimizing the hold profile, consistent parts having minimal defectsand variances in size are produced. Measurements obtained from thenon-time dependent variable or variables can be used as a highlyaccurate measure of when to make process parameter decisions. Further,due to the consistency in molded parts produced when using the optimizedhold profile, the subsequent cure profile may further ensure that moldedparts remain consistent and within tight tolerances (e.g., withintolerances of approximately 2 mm).

BRIEF DESCRIPTION OF THE DRAWINGS

The above needs are at least partially met through provision of one,more than one, or any combination of the approaches for injectionmolding expanding crosslinking polymers described in the followingdetailed description, particularly when studied in conjunction with thedrawings, wherein:

FIG. 1 illustrates an elevation view of an exemplary injection moldingmachine having a controller coupled thereto in accordance with variousembodiments of the present disclosure;

FIG. 2 illustrates an example relationship between a blowing agent and acrosslinking agent over time during injection molding of an expandingcrosslinking polymer in accordance with various embodiments of thepresent disclosure; and

FIG. 3 illustrates an example relationship between screw position,cavity pressure, and melt pressure during an expanding crosslinkingpolymer injection molding cycle in accordance with various embodimentsof the present disclosure.

Skilled artisans will appreciate that elements in the figures areillustrated for simplicity and clarity and have not necessarily beendrawn to scale. For example, the dimensions and/or relative positioningof some of the elements in the figures may be exaggerated relative toother elements to help to improve understanding of various embodimentsof the present invention. Also, common but well-understood elements thatare useful or necessary in a commercially feasible embodiment are oftennot depicted in order to facilitate a less obstructed view of thesevarious embodiments. It will further be appreciated that certain actionsand/or steps may be described or depicted in a particular order ofoccurrence while those skilled in the art will understand that suchspecificity with respect to sequence is not actually required. It willalso be understood that the terms and expressions used herein have theordinary technical meaning as is accorded to such terms and expressionsby persons skilled in the technical field as set forth above exceptwhere different specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

An injection molding process for expanding crosslinking polymers isherein described. Examples of expanding crosslinking polymers includeEVA, which, when polymerized, include any number of blowing agents andany number of crosslinking agents which are activated by a specifiedtemperature. For example, the blowing agents and crosslinking agents maybe activated at temperatures between approximately 160° C. andapproximately 190° C., or preferably, at temperatures betweenapproximately 165° C. and approximately 185° C., and more preferably, attemperatures between approximately 170° C. and approximately 180° C.,which may provide an optimal range for blowing and crosslinking tooccur. Other examples of suitable temperature ranges are possible.

As illustrated in FIG. 1, an injection molding machine 100 that moldsexpanding crosslinking polymers includes an injection unit 102 and aclamping system 104. The approaches described herein may be suitable forvertical press injection molding machines and any other known types ofinjection molding machines. The injection unit 102 includes a hopper 106adapted to accept the expanding crosslinking polymer material in theform of pellets 108 or any other suitable form. In many of theseexamples, the pellets 108 include any number of foaming agents,crosslinking agents, and the like. Other examples are possible.

The hopper 106 feeds the pellets 108 into a heated barrel 110 of theinjection unit 102. Upon being fed into the heated barrel 110, thepellets 108 may be driven to the end of the heated barrel 110 by areciprocating screw 112. The heating of the heated barrel 110 and thecompression of the pellets 108 by the reciprocating screw 112 causes thepellets 108 to melt, thereby forming a molten plastic material 114. Themolten plastic material 114 is typically processed at a temperatureselected within a range between about 110° C. and about 150° C. Thismelt temperature is below an activation temperature of the moltenplastic material 114.

The reciprocating screw 112 advances forward and forces the moltenplastic material 114 toward a nozzle 116 to form a shot of plasticmaterial 114 which will ultimately be injected into a mold cavity 122 ofa mold 118 via one or more gates 120 which direct the flow of the moltenplastic material 114 to the mold cavity 122. In other embodiments, thenozzle 116 may be separated from one or more gates 120 by a feed system(not illustrated). The mold cavity 122 is formed between the first andsecond mold sides 125, 127 of the mold 118 and the first and second moldsides 125, 127 are held together under pressure via a press or clampingunit 124. The mold 118 may include any number of mold cavities 122 toincrease overall production rates. The shapes and/or designs of thecavities may be identical, similar, and/or different from each other.

The press or clamping unit 124 applies a predetermined clamping forceduring the molding process which is greater than the force exerted bythe injection pressure acting to separate the two mold halves 125, 127,thereby holding together the first and second mold sides 125, 127 whilethe molten plastic material 114 is injected into the mold cavity 122. Tosupport these clamping forces, the clamping system 104 may include amold frame and a mold base, in addition to any other number ofcomponents.

The reciprocating screw 112 continues forward movement, causing the shotof molten plastic material 114 to be injected into the mold cavity 122.The mold cavity 122 is heated to a temperature that is higher than theactivation temperature of the molten plastic material 114. For example,the mold cavity 122 may be heated to a temperature between approximately160° C. and approximately 185° C., and preferably, to a temperaturebetween approximately 170° C. and 180° C. As such, a chemical reactionbegins to occur within the molten plastic material 114 as it contactssidewalls of the mold cavity 122. It is understood that walls of themold cavity 122 may be preheated prior to injection the molten plasticmaterial 114, or alternatively, may be rapidly heated to a suitabletemperature as the molten plastic material 114 enters the mold cavity122. Examples of heating techniques that may be used to heat surfaces ofthe mold that define the mold cavity are: resistive heating (or jouleheating), conduction, convection, use of heated fluids (e.g.,superheated steam or oil in a manifold or jacket, also heat exchangers),radiative heating (such as through the use of infrared radiation fromfilaments or other emitters), RF heating (or dielectric heating),electromagnetic inductive heating (also referred to herein as inductionheating), use of thermoelectric effect (also called the Peltier-Seebeckeffect), vibratory heating, acoustic heating, and the use of heat pumps,heat pipes, cartridge heaters, or electrical resistance wires, whetheror not their use is considered within the scope of any of theabove-listed types of heating.

The injection molding machine 100 also includes a controller 140 whichis communicatively coupled with the machine 100 via connection 145, andis generally used to control operation of the injection molding machine100. The connection 145 may be any type of wired and/or wirelesscommunications protocol adapted to transmit and/or receive electronicsignals. In these examples, the controller 140 is in signalcommunication with at least one sensor, such as, for example, sensor 128located in the nozzle 116 and/or a sensor 129 located proximate to anend of the mold cavity 122. The sensor 129 may be located at anyposition within or near the mold cavity 122. It is understood that anynumber of additional sensors capable of sensing any number ofcharacteristics of the mold 118 and/or the machine 100 may be placed atdesired locations of the machine 100.

The controller 140 can be disposed in a number of positions with respectto the injection molding machine 100. As examples, the controller 140can be integral with the machine 100, contained in an enclosure that ismounted on the machine, contained in a separate enclosure that ispositioned adjacent or proximate to the machine, or can be positionedremote from the machine. In some embodiments, the controller 140 canpartially or fully control functions of the machine via wired and/orwired signal communications as known and/or commonly used in the art.

The sensor 128 may be any type of sensor adapted to measure (eitherdirectly or indirectly) one or more characteristics of the moltenplastic material 114. The sensor 128 may measure any characteristics ofthe molten plastic material 114 that is known in the art, such as, forexample, pressure, or temperature, and the like, or any one or more ofany number of additional characteristics which are indicative of these.The sensor 128 may or may not be in direct contact with the moltenplastic material 114. In some examples, the sensor 128 may be adapted tomeasure any number of characteristics of the injection molding machine100 and not just those characteristics pertaining to the molten plasticmaterial 114.

The sensor 128 generates a signal which is transmitted to an input ofthe controller 140. If the sensor 128 is not located within the nozzle116, the controller 140 can be set, configured, and/or programmed withlogic, commands, and/or executable program instructions to provideappropriate correction factors to estimate or calculate values for themeasured characteristic in the nozzle 116.

Similarly, the sensor 129 may be any type of sensor adapted to measure(either directly or indirectly) one or more characteristics of themolten plastic material 114 to detect its presence and/or condition inthe mold cavity 122. In various embodiments, the sensor 129 may belocated at or near an end-of-fill position in the mold cavity 122. Thesensor 129 may measure any number of characteristics of the moltenplastic material 114 and/or the mold cavity 122 that is known in theart, such as, for example, pressure, temperature, and the like, or anyone or more of any number of additional characteristics which areindicative of these. The sensor 129 may or may not be in direct contactwith the molten plastic material 114.

The sensor 129 generates a signal which is transmitted to an input ofthe controller 140. If the sensor 129 is not located at the end-of-fillposition in the mold cavity 122, the controller 140 can be set,configured, and/or programmed with logic, commands, and/or executableprogram instructions to provide appropriate correction factors toestimate or calculate values for the measured characteristic at theend-of-fill position. It is understood that any number of additionalsensors may be used to sense and/or measure operating parameters. Forexample, U.S. patent application Ser. No. 15/198,556, filed on Jun. 30,2016 and published as U.S. Publication No. 2017/0001356, describes asensor positioned prior to the end-of-fill to predict the end-of-filland is hereby incorporated herein by reference in its entirety.

The controller 140 is also in signal communication with the screwcontrol 126. In some embodiments, the controller 140 generates a signalwhich is transmitted from an output of the controller 140 to the screwcontrol 126. The controller 140 can control any number ofcharacteristics of the machine, such as, for example, injectionpressures (by controlling the screw control 126 to advance the screw 112at a rate which maintains a desired melt pressure of the molten plasticmaterial 114 in the nozzle 116), barrel temperatures, clamp closingand/or opening speeds, cooling time, inject forward time, hold profiles,overall cycle time, pressure set points, ejection time, cure profiles,screw recovery speed, and screw velocity. Other examples are possible.

The signal or signals from the controller 140 may generally be used tocontrol operation of the molding process such that variations inmaterial viscosity, mold cavity 122 temperatures, melt temperatures, andother variations influencing filling rate are taken into account by thecontroller 140. Adjustments may be made by the controller 140 in realtime or in near-real time (that is, with a minimal delay between sensors128, 129 sensing values and changes being made to the process), orcorrections can be made in subsequent cycles. Furthermore, severalsignals derived from any number of individual cycles may be used as abasis for making adjustments to the molding process. The controller 140may be connected to the sensors 128, 129, the screw control 126, and orany other components in the machine 100 via any type of signalcommunication known in the art or hereafter developed.

The controller 140 includes software 141 adapted to control itsoperation, any number of hardware elements 142 (such as, for example, amemory module and/or processors), any number of inputs 143, any numberof outputs 144, and any number of connections 145. The software 141 maybe loaded directly onto a memory module of the controller 140 in theform of a non-transitory computer readable medium, or may alternativelybe located remotely from the controller 140 and be in communication withthe controller 140 via any number of controlling approaches. Thesoftware 141 includes logic, commands, and/or executable programinstructions which may contain logic and/or commands for controlling theinjection molding machine 100 according to a mold cycle. The software141 may or may not include an operating system, an operatingenvironment, an application environment, and/or a user interface.

The hardware 142 uses the inputs 143 to receive signals, data, andinformation from the injection molding machine being controlled by thecontroller 140. The hardware 142 uses the outputs 144 to send signals,data, and/or other information to the injection molding machine. Theconnection 145 represents a pathway through which signals, data, andinformation can be transmitted between the controller 140 and itsinjection molding machine 100. In various embodiments this pathway maybe a physical connection or a non-physical communication link that worksanalogous to a physical connection, direct or indirect, configured inany way described herein or known in the art. In various embodiments,the controller 140 can be configured in any additional or alternate wayknown in the art.

The connection 145 represents a pathway through which signals, data, andinformation can be transmitted between the controller 140 and theinjection molding machine 100. In various embodiments, these pathwaysmay be physical connections or non-physical communication links thatwork analogously to either direct or indirect physical connectionsconfigured in any way described herein or known in the art. In variousembodiments, the controller 140 can be configured in any additional oralternate way known in the art.

As previously stated, during an injection molding cycle, the sensors128, 129 are adapted to measure at least one variable related tooperation of the machine 100. During operation, the controller 140commences a hold profile which may be stored in the software 141. Insome examples, the hold profile may be commenced upon the measuredvariable reaching a threshold value. Upon completing the hold profile,the controller 140 will send a signal to the machine that causes themold cavity 122 to open and to eject the part from the mold 118 so thatit can commence the cure profile, where necessary continued expansionand crosslinking occurs to form a structurally sound molded part. Forexample, a structurally sound molded part may be free of divots, dwells,flash, partial fills, burns, tears, minimal imperfections such as sinkmarks and/or swirls on the surface layer, weakness at thickness changes,and may also have uniformity of mechanical properties.

In these examples, the variable or characteristic may be one other thantime (e.g., a cycle, step, or any other time), thus time is not directlymeasured and used to determine the length of the hold profile, andaccordingly, time is not directly measured and used to determine when toeject the part. Rather, the variable or characteristic relies on anothervalue or indicator as a determining factor for part readiness. The useof one or more non-time dependent variables is advantageous becauseduring successive runs, even with the same supply of pellets 108,variations in pellet quality, catalyst stability, ambient conditions, orother factors may influence the cross-linking of the polymer materialfrom shot-to-shot. While a time-dependent process may providesatisfactory parts most of the time, a system that determines ejectionreadiness based on one or more non-time dependent variables ispreferable, as this provides a more accurate assessment for eachindividual shot or run of the molding system.

Turning to FIG. 2, which illustrates an example relationship between theblowing agent and the crosslinking agent of the expanding crosslinkingpolymer over time, during the injection molding process, the blowingagent first activates at a given temperature and begins to react overtime. Generally speaking, the blowing agent, depicted by the solid linein FIG. 2, will cause the part to expand, and thus will at leastpartially dictate the part size. At approximately the same point thatthe blowing agent is activated, the crosslinking agent, depicted by thedashed line in FIG. 2, activates and begins to form structural bondswithin the polymer. Both the blowing agent and crosslinking agentgenerate exothermic reactions, thus they generate heat as the reactionadvances, which in turn causes the blowing and crosslinking agents tocontinue their respective chemical reactions. When the blowing processconcludes, the reaction will stop emitting heat. At this point,crosslinking continues until the part is sufficiently formed, meaningthe molten plastic material 114 is no longer in a flowable state.

Referring again to FIG. 1, upon the molten plastic material 114substantially filling the mold cavity 122, a hold profile is commenced.During the hold profile, which may commence upon the measured variable(which can be measured by any of sensors 128 and/or 129) reaching afirst threshold value, additional molten plastic material 114 isrestricted from being injected into the mold cavity 122. This may occurby shutting off the supply of molten plastic material 114, oralternatively or in combination, by controlling movement of the screw112. Additionally, the mold cavity 122 is held closed during the holdprofile. Upon the measured variable (which can be measured by any ofsensors 128 and/or 129) reaching a second threshold value, controller140 causes the hold profile to end, whereby the mold cavity 122 isopened and the part is ejected from the mold 118 and the cure profile tocommence.

Turning now to FIG. 3, which represents an example expandingcrosslinking polymer injection molding cycle 300, the measured variablemay reach first and second threshold values. Line 302 depicts theposition of the screw 112 under a certain injection pressure (i.e.,5,000 psi) once the cavity pressure is built to a desired and/ordesignated trigger point. As an example, the pressure can decrease fromapproximately 5,000 psi to approximately 2,000 psi at this point. Inthis example, during injection of the expanding crosslinking polymer,melt pressure, which is depicted by line 304, is first increased andthen held to a substantially constant value. Accordingly, the sensor 128may be a pressure sensor disposed within, on, and/or near the screw 112.As a non-limiting example, the melt pressure may be betweenapproximately 0 psi and approximately 11,000 psi. Other examples ofsuitable melt pressures are possible. Further, it is understood that insome examples, the melt pressure may not be held to a substantiallyconstant value.

In FIG. 3, line 306 depicts the measured variable as a cavity pressurevalue. Accordingly, in this example, the sensor 129 may be a pressuresensor disposed within, on, or near the mold cavity 122. In theillustrated example, in region I, the sensor 129 measures a cavitypressure that exceeds the first threshold value. As previously noted, insome examples, during the injection molding process, the mold cavity 122can be essentially completely filled with molten plastic material 114.

In this example, the measured cavity pressure value is defined as acavity pressure greater than a nominal value, which may be at leastpartially caused by the molten plastic material 114 completely fillingthe mold cavity 122 and exerting a pressure on the cavity walls. Theincrease in cavity pressure may additionally or alternatively be causedby the expansion of the molten plastic material 114 within the moldcavity 122. It is understood that in some examples, the first thresholdvalue may be any desired quantity. For example, the first thresholdvalue may be a distinct cavity pressure value, such as, approximately100 psi. Other examples are possible.

Upon the sensor 129 measuring a cavity pressure value exceeding thefirst threshold value, the controller 140 commences the hold profile. Asillustrated by line 304 in FIG. 3, the melt pressure is then adjusted(for example, reduced). In the illustrated example, the melt pressure isagain held to a substantially constant value, such as, for example,between approximately 500 psi and approximately 3,500 psi. Otherexamples are possible. This pressure is maintained by controllingmovement of the screw 112 to a hold pressure measured at the nozzle bythe sensor 128.

At region II, as the melt pressure is maintained, the measured cavitypressure increases as the molten plastic material 114 begins to blowand/or expand. Upon the sensor 129 measuring a cavity pressure valuethat exceeds the second threshold value, the hold profile is completed,and the controller 140 causes the part to be ejected from the moldcavity 122. As an example, the second threshold value may be a distinctcavity pressure value, such as, between approximately 100 psi andapproximately 2,000 psi. Other examples are possible. This secondthreshold value is indicative of the expanding crosslinking polymericpart being sufficiently structurally sound to complete the remainder ofits expansion and crosslinking outside of the mold cavity. At thispoint, the mold cavity 122 is opened, thus the melt pressure drops toapproximately 0.

In some examples, the sensor 129 is a temperature sensor that measures atemperature value. Accordingly, in these examples, the first thresholdvalue may be a cavity temperature value that is representative of themold cavity 122 being substantially completely filled. For example, thefirst threshold temperature value may be between approximately 168° C.and approximately 176° C. Other examples are possible. Similarly, inthese examples, the second threshold value may be a cavity temperaturevalue that is representative of the molten plastic material 114 beingsufficiently structurally sound for ejection. In these examples, thecavity temperature may plateau or decrease at a point when the part isready to be ejected from the mold cavity 122. As a non-limiting example,the second threshold temperature value may be between approximately 160°C. and approximately 180° C. Other examples are possible.

Because the mold cavity 122 is substantially completely filled (e.g.,between approximately 95% and approximately 99% fill) prior tocommencement of the hold profile, and because pressure is applied to themolten plastic material 114 thereby holding it against the heated wallsof the mold cavity 122, heat is uniformly distributed or transferred tothe molten plastic material 114 due to the increased surface contact.Advantageously, the blowing and crosslinking agents will activate moreuniformly, thus forming more cohesive bonds.

So configured, the hold profile can be described as the combination ofregions I and II in FIG. 3. The injection molding machine 100 does notcontemplate the actual duration of time required to commence the holdprofile, and rather, the machine 100 operates in a closed loop moldholding pattern. So configured, molded parts have more consistent partsizes and appearances, as well as a uniform skin layer due to consistentheat transfer. Further, not only will particular parts have consistentdimensions, the hold profile helps to ensure reliability and consistencyacross a range of sizes of parts, which has been particularlychallenging with respect to expanding crosslinking polymer articles.Further still, the hold profile provides better control over theinjection process, allowing the part to dictate when the cavity is fulland ready to be ejected. In some examples, using the hold profile candecrease the overall cycle time due to a reduced cure time.Additionally, the use of the hold profile can generate parts having moreuniformity in cell structure due to free radical molecules becomingaligned. These parts will have minimal imperfections such as sink marksand/or swirls on the surface layer. As such, the hold profile makes amore consistent and stable dimensioned part, with consistent physicalproperties.

At region III, the controller 140 commences a cure profile. Asillustrated in FIG. 3, the cavity pressure will ultimately plateau asthe part ceases to further expand. Upon the sensor 129 measuring acavity pressure value that exceeds a third threshold value, the cureprofile is completed, and the part is ejected, removed from the cavity122 or the entire machine 100, and transferred to a stabilization tunnelwhere curing occurs. As an example, the third threshold value may be adifferent cavity pressure value, such as, between approximately 2,000psi and approximately 4,000 psi. Other examples are possible.Alternatively, the third threshold value may be a predetermined rate ofchange in pressure values, which may indicate that the pressure is nolonger increasing. Other examples are possible. This third thresholdvalue is indicative of the expanding crosslinking polymeric part beingessentially fully formed and ready for further processing. By using thethird threshold value to determine the duration of the cure profile, themachine 100 will not prematurely eject parts that have not fully cured.Additionally, the machine 100 reduces inefficiencies by usingunnecessarily long cure times, which can consume unnecessary power andreduce overall yields of the machine.

In some examples, the pressure value may be measured directly using apressure transducer. In this example, the “head” of the transducercontacts the molded material, and a pressure is sensed through thiscontact. In other examples, the pressure value may be measured at asub-surface location via a pressure transducer in a blind pocket that isseparated from the cavity by a thin membrane of mold material. In yetother examples, the pressure value may be indirectly measured via afloating pin disposed in a hole in the cavity that transfers load fromthe cavity to the pressure sensor. In other examples, force from thesub-surface and/or indirect measurements may be augmented through alever mechanism or other machine. Other examples are possible.

In other examples, the cure profile may be commenced for a fixed,predetermined time necessary for the part to be fully cured. Forexample, the cure profile may be programmed to last betweenapproximately 100 seconds and approximately 450 seconds. Other examplesare possible. The machine 100 is capable of using a fixed period of timefor the cure profile due to the use of the optimized hold profile, whichforms consistent parts having uniform characteristics, such as internalcrosslinking and bond strength. This uniformity at the onset of the cureprofile will result in continued uniformity during the cure profile.

Those skilled in the art will recognize that a wide variety ofmodifications, alterations, and combinations can be made with respect tothe above described embodiments without departing from the scope of theinvention, and that such modifications, alterations, and combinationsare to be viewed as being within the ambit of the inventive concept.

The patent claims at the end of this patent application are not intendedto be construed under 35 U.S.C. § 112(f) unless traditionalmeans-plus-function language is expressly recited, such as “means for”or “step for” language being explicitly recited in the claim(s). Thesystems and methods described herein are directed to an improvement tocomputer functionality, and improve the functioning of conventionalcomputers.

What is claimed is:
 1. A method for determining whether an expandingcrosslinking polymer part is ready to be ejected from a mold cavityduring an injection molding cycle, the method comprising: injectingmolten expanding crosslinking polymer into a mold cavity; measuring atleast one non-time dependent variable during the injection mold cycle;commencing a hold profile for the expanding crosslinking polymer part;upon completing the hold profile, ejecting the molded part from the moldcavity; and commencing a cure profile for the molded part.
 2. The methodof claim 1, wherein commencing the hold profile comprises: upon themeasured at least one non-time dependent variables reaching a firstthreshold value, commencing the hold profile; restricting additionalmolten expanding crosslinking polymer from being injected into the moldcavity; and upon the measured at least one non-time dependent variablesreaching a second threshold value, terminating the hold profile.
 3. Themethod of claim 2, wherein the first threshold value is indicative ofthe mold cavity being substantially full of molten expanding crosslinking polymer.
 4. The method of claim 2, wherein the second thresholdvalue is indicative of the part being structurally sound.
 5. The methodof claim 2, wherein the measured at least one non-time dependentvariable comprises a cavity pressure value.
 6. The method of claim 1,wherein the hold profile commences at a substantially constant pressure.7. The method of claim 1, wherein commencing the cure profile comprises:further measuring a different non-time dependent variable; and upon themeasured different non-time dependent variables reaching a thirdthreshold value, terminating the cure profile.
 8. The method of claim 7,wherein the third threshold value is indicative of the part beingstructurally sound.
 9. The method of claim 7, wherein the measureddifferent non-time dependent variable comprises a pressure value. 10.The method of claim 1, wherein commencing the cure profile comprisesallowing the part to cool for a predetermined amount of time.
 11. Anexpanding crosslinking polymer injection molding system comprising: aninjection molding machine comprising an injection unit and a moldforming a mold cavity, the injection unit adapted to receive and injecta molten expanding crosslinking plastic material into the mold cavity toform a molded part; a controller adapted to control operation of theinjection molding machine; and one or more sensors coupled to theinjection molding machine and the controller; wherein at least one ofthe one or more sensors is adapted to measure at least one non-timedependent variable during the injection mold cycle, wherein thecontroller is adapted to commence a hold profile for the expandingcrosslinking polymer part and is further adapted to cause the moldedpart to be ejected from the mold cavity upon completing the hold profileand commence a cure profile for the molded part.
 12. The system of claim11, wherein the controller commences the hold profile by commencing thehold profile when the measured at least one non-time dependent variablereaches a first threshold value, restricting additional molten expandingcrosslinking polymer from being injected into the mold cavity, andterminating the hold profile when the measured at least one non-timedependent variables reaches a second threshold value.
 13. The system ofclaim 12, wherein the first threshold value is indicative of the moldcavity being substantially full of molten expanding cross linkingpolymer.
 14. The system of claim 12, wherein the second threshold valueis indicative of the part being structurally sound.
 15. The system ofclaim 12, wherein the measured at least one non-time dependent variablecomprises a cavity pressure value.
 16. The system of claim 11, whereinthe hold profile commences at a substantially constant pressure.
 17. Thesystem of claim 11, wherein commencing the cure profile comprises:measuring a different non-time dependent variable; and upon the measureddifferent non-time dependent variables reaching a third threshold value,terminating the hold profile.
 18. The system of claim 17, wherein thethird threshold value is indicative of the part being structurallysound.
 19. The system of claim 17, wherein the measured differentnon-time dependent variable comprises a pressure value.
 20. The systemof claim 11, wherein commencing the cure profile comprises allowing thepart to cool for a predetermined amount of time.